A bypass valve according to the related art bypasses an oil such that the oil does not pass through an oil cooler at or below a preset oil temperature, and passes the oil through the oil cooler if the oil temperature exceeds the preset oil temperature so that overheating of the oil can be prevented. This configuration is provided to improve fuel consumption by reducing hydraulic pressure losses generated when the oil passes through the oil cooler.
However, as cooling water introduced into the oil cooler raises the temperature of the oil introduced into the oil cooler at an initial stage of vehicle startup, an oil lubrication effect deteriorates and a fuel efficiency effect is reduced when a general one-stage thermostat valve is applied.
Accordingly, by applying two one-stage thermostat valves, rapid rising of the oil temperature is induced by supplying the oil to the oil cooler at a low temperature. A pressure difference reduction effect is shown by operating a primary thermostat valve and bypassing the oil through the oil cooler if the temperature of the oil reaches a low temperature setting temperature, and the oil is supplied to the oil cooler again by operating a secondary thermostat valve if the temperature of the oil reaches a high temperature setting temperature, so that the coil can be efficiently cooled.
However, to achieve this, two first stage thermostat valves should be connected in series. Accordingly, manufacturing costs increase and the size and volume of the product also increases.
The description provided above as a related art of the present disclosure is just for helping in understanding the background of the present disclosure and should not be construed as being included in the related art known by those skilled in the art.